1. Field of the Invention
The present invention relates to a driving method of a display device, in particular, a driving method of a display device using a time gray scale method.
2. Description of the Related Art
In recent years, research and development of an active matrix display device using digital video signals have been actively carried out. There are, for example, a light receiving display device like a liquid crystal display (LCD) and a self-light-emitting display device like a plasma display in such an active matrix display device. As a light-emitting element used for the self-light-emitting display device, an organic light-emitting diode (OLED) has been attracting attention. The OLED is also referred to as an organic EL element, an electro luminescence (EL) element, or the like (a display using an EL element is referred to as an EL display). The self-light-emitting display device using the OLED or the like has advantages such as higher visibility of pixels than that of a liquid crystal display, and fast response speed without requiring a backlight. The luminance of the light-emitting element is controlled by the value of a current flowing through the light-emitting element.
It is known that a time gray scale method is used as a method for displaying gray scales with the use of digital video signals in such an active matrix display device.
The time gray scale method is a method for displaying a gray scale by controlling the length of a light-emitting period or the frequency of light emission. In other words, one frame period is divided into a plurality of sub-frame periods, each of which is weighted with respect to the frequency of light emission, a light-emitting time, and the like, and then the total weight (the sum of the frequency of light emission and the sum of the light-emitting time) is differentiated in each gray scale, thereby displaying a gray scale. As an example, FIG. 34 shows a case where one frame is divided into five sub-frames SF1 to SF5 so that the ratio of lighting periods of these sub-frames is weighted to be 20:21:22:23:24. FIG. 35 shows a relation between lighting/non-lighting selective patterns of these sub-frames and gray scales. As apparent from FIGS. 34 and 35, by controlling lighting/non-lighting of the sub-frames SF1 to SF5, 32 gray scales of 0 to 31 can be displayed (a gray scale of 1 represents a minimum unit of gray scale change). Since 1 bit is necessary to order lighting/non-lighting of each sub-frame, a 5-bit digital signal is necessary to control the five sub-frames SF1 to SF5. In general, by controlling M sub-frames each having a weighting of power-of-2 with the use of M-bit digital video signals, display of 2M gray scales (that is, a gray scale is 0 to 2M−1) can be performed. In this specification, it is to be noted that, in such a manner, a time gray scale method for performing gray scale display by using a plurality of sub-frames which have different weightings is referred to as a binary code time gray scale method. A digital signal bit which controls a sub-frame that is weighted large (for example, SF5) is referred to as a high-order bit, and a digital signal bit which controls a sub-frame that is weighted small (for example, SF1) is referred to as a low-order bit. It is to be noted that the sub-frames may not necessarily be weighted to be power-of-2 and not all sub-frames have to be weighted differently. The weighting (a lighting period or the frequency of flickering) of one sub-frame may be less than or equal to a value of the total weightings of the sub-frames of which weighting is smaller (that is, a lower-order weighting), to which 1 is added. Accordingly, all gray scales can be displayed continuously. For example, when the length ratio of a lighting period of each sub-frame is regarded as 1:1:2:3, all gray scales of from 0 to 7 can be displayed continuously.
In the display device using such a binary code time gray scale method, a false contour (also referred to as a pseudo contour) may be perceived at a portion where the gray scale changes smoothly originally without generating a boundary, when displaying a moving image. It is known that a pseudo contour is likely to be generated when pixels, of which lighting patterns differ largely like a case where one adjacent pixel has a gray scale of 15 and the other has a gray scale of 16, are adjacent to each other. In order to reduce such a pseudo contour, various countermeasures have been proposed (see References 1 to 8: Japanese Patent No. 2903984, Japanese Patent No. 3075335, Japanese Patent No. 2639311, Japanese Patent No. 3322809, Japanese Published Patent Application No. H10-307561, Japanese Patent No. 3585369, Japanese Patent No. 3489884, and Japanese Published Patent Application No. 2001-324958).
For example, Reference 2 discloses that 7 sub-frames having almost the same weighting (high-order sub-frames) are controlled with high-order 7 bits of a 12-bit digital signal that displays gray scales, and a plurality of sub-frames of which weightings are performed in accordance with a binary digit is controlled with the other 5 low-order bits, for example. Here, the seven high-order sub-frames are continuously provided in one frame period, and the high-order sub-frames are sequentially lighted cumulatively as the gray scales increase. In other words, the high-order sub-frames that are lighted at small gray scales are lighted also at large gray scales. Such a gray scale method is referred to as an overlapping time gray scale method.